With the increasing demand for energy and the pressing issues associated with CO2 emissions, nuclear power is once again becoming an attractive alternative for electrical production. The generation-IV reactors which are under development will run at much higher temperatures and with a much greater burn-up of the fuel than the current generation of nuclear reactors. A consequence of this higher working temperature and greater burn-up is a need for new fuel formulations.
There are many materials that may fit the requirements of the generation-IV reactors. Oxides are relatively simple to produce and are the current materials of choice for conventional reactors. Unfortunately, oxide fuels have several inherent limitations, which include a relatively low fissile density that reduces the breeding ratio and poor thermal conductivity that restricts the linear heating rate. Of the alternatives, nitrides represent the best combination of properties with the potential to solve these problems.
Uranium nitride has many favorable fuel properties, such as a high fissile density, a high melting point similar to that of oxide fuel and high thermal conductivity similar to that of metal fuel. While uranium nitride (UN) has many of the properties that would make it an excellent reactor fuel, it has failed to make the leap to practical systems due to the difficulty in its synthesis. In particular, the inclusion of carbon from the currently favored carbothermic reduction routes to UN is a major issue in the production of nitride fuels. The carbothermic synthesis relies on the conversion of the uranium carbide to the nitride at high temperatures. The high temperatures required for the UN production have unfortunate side effects in that the low vapor-pressure actinides, particularly americium, become volatile leading to serious contamination issues.
GB 1,186,630 discloses a process for the production of uranium nitride, plutonium nitride, or mixtures thereof, by reaction of uranium or plutonium tetrafluorides or trifluorides by high-temperature ammonolysis into the corresponding higher nitride or mixture of higher nitrides, which is then dissociated in vacuo into the corresponding mononitride or mixture of mononitrides. Optionally, zirconium nitride can be included in the admixture.
U.S. Pat. No. 3,953,355 discloses a process for the preparation of actinide nitrides from massive actinide metal, massive being a single piece of metal having a mass of 0.1 kg or more. The process involves partially hydriding the massive metal and simultaneously dehydriding and nitriding the dehydrided portion. The process is repeated until all of the massive metal is converted to a nitride.
U.S. Pat. No. 5,128,112 discloses a process of preparing an actinide nitride, phosphide, sulfide or oxide, by admixing an actinide organometallic precursor with a suitable solvent and a protic Lewis base selected from ammonia, phosphine, hydrogen sulfide and water, and heating the mixture until the actinide compound is formed.
B. N. Wani et al. report in Fluorination of Oxides of Uranium and Thorium by Ammonium Hydrogen Fluoride. J. Fluorine Chem. 44 (1989) 177-185, that UO2, U3O8 and ThO2 were fluorinated by NH4HF2 at room temperature to produce (NH4)4UF8·2H2O, (NH4)3UO2F5·H2O and (NH4)4ThF8·2H2O, respectively.
There remains a need to provide low temperature methods of preparing actinide nitrides for use as nuclear fuels.